This resolves according to the readout the monthly tab for the active users traders chart of the Manifold stats page at the end of 2025. An "active user" is currently described as of market creation as

An active user is a user who has traded in, commented on, or created a question.

If that definition changes, I will do my best to either get an accurate number in the spirit of the original meaning, or N/A the market.

UPDATE: This chart now seems to be called "Active traders", so per my criterion above I am updating the title.